Question

What is band theory of metals?

Answer 1

Band theory explains the conductivities of conductors, insulators and semi conductors.
The valence band can be treated as the valence shell that contain free electrons. The interaction between the valence band and the conduction band decide the conductivity of a particular material.
If the energy gap between the two shells is negligible the electrons can jump easily from valence band to conduction band. Hence they behave as good conductors.
When the energy gap is small that the electrons can jump from valence band to conduction band on providing little energy then these type of solids behave as semi conductors.
When this gap is large enough for the electrons to excite themselves from valence band to conduction band then they act as insulators. this large energy gap between the conduction band and valence band in insulators is known as forbidden zone...